Below, you will find links to articles with lists of companies that supply VCU's (verified carbon units) or CER's (certified emission reductions). Both types are basically the same (CO² emission reductions from either planting trees, or other projects in which the emission of carbon dioxide has been avoided). VCU's and CER's differ only in that the organisation that performs the control is different, and the first tends to focus on regular consumers (private people) whereas the latter focuses mainly on companies that actually need to offset a part of their emissions, as required by law.
The best off-sets you can buy are those of REDD projects (see here), mainly as in these projects, existing forests are left standing, meaning the local fauna/flora in them are kept intact -by comparison, regular tree planting projects may involve just making new forests on cleared land, using non-native tree species-.
Other projects still are projects in which emissions have just been avoided using other means (for example by making existing power plants made less polluting, ...)
Regardless however of which project you prefer to buy the credits from (often, you can select this yourself !), the most important is that you indeed offset all your emissions you generated anually (rather than not doing it at all).
Links to articles with carbon offset provider lists:
* Endscarbonoffsets
* UNFCCC's CDM site
* CORE: offset providers
PS: the family featured in the spreadsheet used companies as Trees for all , CoolEarth, World Land Trust, ...