None of the dining plans include gratuities these days.
I'm working from memory, so I'm sure someone will come along soon and correct any errors.
There are essentially three dining plans: counter service (quick service in WDW terms), DDP (includes 1 QS, 1 TS & 1 snack daily), and DxDDP (Deluxe Dining Plan), which includes three TS meals daily--or you can elect to use QS, if you choose.
There also is a wine package available as an add-on--not sure if applies only to the DDP and DxDDP. It would make sense if it did--most QS venues don't offer wine, except in EPCOT. I believe you can purchase any level dining plan at any of the WDW resorts, but I could be wrong!
Free dining is offered by WDW during slow times of the year if bookings are down--or they expect them to be. It usually encompasses the latter part of August and September, sometimes a few days into October. They usually have specific booking windows--I think the US window for 2009 free DDP is about to close--if it hasn't already. I haven't been paying too much attention to it.
Is it worth purchasing the dining plans? If you think of it as a prepaid meal plan, it can be worthwhile. If it's used intelligently, it's possible to save a few dollars each day over strictly OOP. But it can be rather restricting, as you have to make the TS ADRs well in advance and there frequently isn't much wiggle room to change plans at the last minute--or even after arriving at WDW! It's really a matter of personal preference.