The Omni will actually be the city's second tallest, besting D. R. Horton Tower but falling just under Burnett Plaza.
A new parking garage with ground-floor retail and an ungarage-like facade is going up next to the Omni.
The Cassidy is a planned new condo building with ground floor retail space for Sundance Square. It will be 22 stories and go on the southeast corner of 3rd & Throckmorton on a quarter-block site currently used for parking. It will share the block with the Sanger Lofts (the old Sanger and Fakes Buildings) and the small retail buildings on the block. It has been designed by David M. Schwarz (of Bass Hall, Carnegie, Sundance & Palace Theaters, and many other projects). There is no set timetable for construction, but it is definitely planned. It will be another product of Sundance Square, and hence of the Bass family. Technically, as I understand it, initial construction for the building is already complete - when the Sanger Building was turned into lofts and the Sundance West apartment building (also by Schwarz) was built, a two-story underground parking garage was built that stretched under 3rd Street between Sundance West and Sanger. That garage already has the foundation for the Cassidy as part of its structure underground.
The Carnegie has topped out and initial facade work is underway. This building is also a David M. Schwarz design.
Wagner Oil is building the Caceria Building, a new 7-story office building with ground-floor retail on the northwest corner of 5th & Commerce across from the Bass Performance Hall and the redeveloped Embassy Suites Hotel. It has topped out and significant facade work is complete. Wagner also owns 500 Commerce next door on the same block and is remodeling that building's ground floor to add more retail space. The building is designed by Gideon Toal.
A new four-story office building, Cantey Hangar Plaza, is being constructed at 7th & Burnett between 500 West 7th and Burnett Plaza. The building has topped out and facade work is underway. The building is also a Gideon Toal design.
Trinity Terrace City Tower is a retirement apartment tower alongside Trinity Terrace's original '70s-vintage apartment tower on downtown's Upper West Side. The building has topped out and facade work is underway.
Klabzuba Oil is building a new four-story headquarters office building on downtown's Upper West Side at 100 Lexington. The building is a design of Cunningham Architects.
The parking lot on the block with the First Christian Church in downtown (between the Fort Worth Club and The Tower) has been sold to a developer planning an office tower on the site. Timetable unknown.
Lancaster Avenue in downtown has been narrowed back to four lanes with wide sidewalks following the removal of the I-30 Overhead. The other four lanes of the old widened Lancaster have been removed and the surplus land is being sold off to developers wishing to build urban infill along Lancaster.
The historic Texas & Pacific Warehouse on Lancaster is planned to be converted to 300+ loft apartments with ground-level retail, with work beginning this year. This would complete the redevelopment of the old T&P Complex, with the historic passenger terminal having already been redeveloped into very successful loft condos.
Knights of Pythias redvelopment - restoration and redevelopment of historic 1925 Knights of Pythias building on downtown's east side in Hillside neighborhood.
The Ruins - Planned 26 story condo tower on downtown's Upper West Side by the Pier 1 tower. Designed by local architect Ken Schaumburg. Glass tower covered in Greek-inspired ruins. Current timetable shows a Winter 2009 completion date.
In Uptown, several large developments are underway:
Pecan Place townhomes - many new three-story infill townhomes.
Palisades townhomes - many new three-story infill townhomes on the Trinity Bluff.
Lincoln Trinity Bluff - four-story infill apartments on the Trinity Bluff.
Villa De Leon - Seven-story luxury condo building on the Trinity Bluff.
Trinity Bluff Phase 4 & 5 - Several hundred new infill apartments on the Trinity Bluff.
LaGrave Field mixed-use - Planned mixed-use development replacing parking lots around LaGrave Field (baseball field across the river from downtown that is home of the Fort Worth Cats).
Tarrant County College downtown campus - A futuristic TCC campus on both the downtown and Uptown sides of the Trinity River. Designed by Bing Thom of Vancouver. The campus cantilevers over the river and the two sides are linked by a pedestrian bridge.
Several developments are underway in the Cultural District immediately west of downtown. These include:
Montgomery Plaza - redevelopment of historic '20s Montgomery Ward building. Ground floor retail already in place, high-end loft condos now under construction on upper floors. 70%+ sold already.
Presidio Townhomes - planned for vacant lot across from Montgomery Plaza. Three story infill townhomes.
So7 - South of Seventh - large infill development across from Montgomery Plaza. Includes 59 townhomes, 65 condos, 180 lofts, extensive retail space, and a Marriott Residence Inn. Final phase, The Shops & Lofts at So7, underway now.
The Stayton - on So7 property, a luxury retirement condo development which will consist of three 12-story towers.
West 7th by Cypress Equities - 345 apartments over retail, a 150 room hotel, a movie theater, and new seven-story office buildings in an infill project on five blocks bounded by 7th, University, Morton, and Foch.
Museum Place - 11-acre urban infill development across from the Modern Art Museum, Kimbell Art Museum, and Amon Carter Museum. Features over 500 apartments, 40 condos, 173,000 square feet of ground-level retail space, 130,000 square feet of Class A office space, and an Aloft Hotel.
Several developments are underway or completed in Fort Worth South, the urban neighborhood immediately south of downtown. These include:
Baylor Hospital expansion
Mixed-use condo/retail building at Magnolia & Henderson
Restoration and redevelopment of historic Max Mehl Building at Magnolia & Henderson
Oleander Walk - conversion of Oleander Street to a pedestrian thoroughfare
Fairmount Lofts - Lofts on Fairmount at Oleander. Phase 1 complete, Phase 2 getting underway soon.
Texana Townhomes - infill towhomes at Adams & Oleander. Phases 1 and 2 complete.
Oleander Place townhomes - infill townhomes at Adams & Oleander. Phase 1 underway.
Southside Associates Mixed-Use lofts - planned loft development on Oleander.
LaSalle Apartments restoration - Restoration of historic LaSalle Apartments on College in Fairmount. Complete.
Magnolia Green - mixed-use development on Lipscomb between Magnolia & Rosedale. Several new infill office buildings, luxury infill townhomes, a public garage, and a public park.
Lofts of St. Mary's - redevelopment of St. Mary's church gymnasium into mixed-use lofts.
Oleander Lofts - mixed-use loft/office development on Oleander next to Baylor Hospital. Complete.
Pennsylvania Lofts - mixed-use loft/office development on Pennsylvania next to Harris Hospital. Complete.
New medical office building - infill going in at Pennsylvania & Hemphill I believe.
Sawyer Grocery redevelopment - restoration and redevelopment of historic Sawyer Grocery Store on South Main into apartments over retail. Underway.
Former Fort Worth ISD property redevelopment - Multiblock mixed-use retail/condo infill planned for South Main & Pennsylvania.
Whew. There's more, but I need to take a break from typing.

I will add renderings where I have them.