There's a free, open source project called php open tracker. It had a plugin for conversions; I just downloaded it but it looks promising. I've also developed one that just writes a cookie upon first visit and then retrieves it and writes it to a db when they purchase... It's pretty simple to make but all the reports can be tedious to write in php... The best solution would be combined stats and conversions, something that can get expensive, especially on hosted products.