-This is a quick reference for new Tacoma owners on two oft-discussed TSBs they should be aware of
-A more comprehensive list of TSBs (thanks to Angus_NB)
A TSB is a Technical Service Bulletin. The two most talked about TSBs regarding 2nd gen Tacomas are the rear spring TSB and the tailgate TSB.
Rear Spring TSB
If you are still under warranty (36k/36mo) and have a 2005-2010 Tacoma 4x4 or PreRunner (including TRDs), you are eligible for the rear spring TSB, which replaces your shocks with the Bilstein shocks normally given on the TRD package and replaces your 3-leaf springs in the rear with 4-leaf springs. This is to help with bottoming out and sagging that's experienced when a heavy load is placed in the bed. The TSB is specified for the 2005-2010 models.
If you approach your service dept about getting this TSB done, it's good to have the TSB printed out in-hand when you show up.
Get the TSB to print out here.
See this post & discussion for tips on getting it approved without too much hassle.
Bent Tail Gate TSB
On 2005-2008 models, there is a TSB to strengthen the tailgate which can be bent under certain conditions. Note this has been fixed in 2008s made after a certain date (details in the pdf below).
Here is the PDF for the tail gate TSB: BO018-07.pdf
(thanks to Flycatcher for assisting with this)