Thea Render is going to incorporate GPU rendering in v1.3 which will probably come before the end of the year http://www.thearender.com/downloads/TheaRenderRoadmap.pdf.
A list of released and announced GPU renderers:
1. V-Ray GPU (Chaos Group)
2. iray (mental images)
3. SmallLuxGPU (LuxRender)
4. Octane Render (Refractive Software)
5. Arion Render (Random Control/FryRender)
6. Thea Render
7. SHOT using iray (Bunkspeed)
8. RTT Powerhouse and RTT DeltaGen using iray (Realtime Technology)
9. Indigo Render also announced plans for GPU acceleration
UPDATE 2 (Sep 29):
10. finalRender (cebas Visual Technology) http://www.cebas.com/?pid=hot_news&nid=378
11. Artisan using OptiX (LightWorks) http://architosh.com/2010/07/sig-lightworks-unveils-power-of-optix/
12. Zeany using OptiX (Works Zebra)
I bet Maxwell and Modo will quickly follow.