I also use O&O Defrag and find it great. It even has generally low requirements and I can continue my work as it defrags without any problem. Tune Up's defrag is good too.
I use Diskeeper Pro 2008 and i really like it. Runs using only minimal resources and I can use the PC as I want to. I've even played Oblivion, COD4 etc with it running in background mode, without a hitch!
