We've started using Trac[^] recently. Inital impressions are good. Easy enough to set up, reasonably easy to customise (I'm talking page style rather than content - the content consists of a wiki for static, non-ticket information in addition to theticketing system). It interfaces to Subversion nicely (and allegedly to other systems, although I've only used SVN). Of course, it's pretty much directly equivalent to RedMine (I hadn't noticed you saying you already used that internally at first), although (from my impressions) more mature and better documented. Couldn't you just (using some clever URL rewriting or something) just expose the issue tracking part of RedMine to your customers without them being aware of the rest of RedMine? If I were in your place, I wouldn't want the hassle of two separate issue tracking systems and having to translate between the two.