Both DD-WRT and Tomato can do it on supported hardware, but there are typically better ways to increase range. I've found AP placement and antenna choice/direction have the most impact.

Still, the hardware itself makes a huge difference...I've found the Buffalo WHR-HP-G54 (readily available on eBay) to have far superior signal strength to any of the Linksys models I've used, and it's both DD-WRT and Tomato compatible. There are tons of Linksys models and I've not tried them all, but still...if you don't already have a Linksys, the Buffalo is relatively cheap and gets great reviews.