California also has a website that lists free state tax prep. I've never heard of most of those companies, so I'm a little hesitant to use them. The also have a state-developed "calfile" site that is free to most all Californians (here's a less then ecstatic review from 2005). The ultimate cheap option is to use one free site for your fed taxes and one for your state taxes, but as you will see below that only saves you $10, vs. the cheapest reputable pay sites. Doing both taxes on one site is nice because you don't have to double-enter anything.
If you can't do it for free, how much does it cost from the major providers to do both if you make less than $55k (unless otherwise noted)? (prices as of 2-5-09)
H & R block: $30
Turbo Tax Freedom Edition (if you make less than $30k) $10
Tax slayer $10
Complete Tax: $15 (if you make less than $31k).
(note to get these prices you may have to follow the link from the IRS website).
I've used all of these, over the years, and they are all OK. I've been most please with Turbo Tax and H&R block over the years, interface wise. But none of them stand out as being all that different, really.