I have used subversion as a source control repository for some time (without getting into the religious argument - I find it does the job really well); and recently I have started to use the DiffMerge tool from SourceGear (http://www.sourcegear.com/diffmerge/index.html). I know there are loads of tools out there, I have found that this one does a reasonable job of just about everything (that's pretty much why I use it!). I particularly like the implementation of the three file view for conflict resolution.
It doesn't take too much setting up to use with TortoiseSVN. Open the Settings dialog from the TortoiseSVN Settings menu.
In the External Programs section select Diff Viewer and select the External radio and supply the command line argument:
path\DiffMerge.exe /t1=Mine /t2=Original %mine %base
Make sure you supply a valid path to the installed DiffMerge product! When displaying changes this will display your version changes in the left panel with base in the right.
To get the three panel merge select the Merge Tool in the External Programs section, and as before select the External radio and supply the following command line argument:
path\DiffMerge.exe /t1=Mine /t2=Base /t3=Theirs /r=%merged %mine %base %theirs
This will display three columns with your version, followed by the base version followed by the repository version - allowing you to merge changes from both sides into the centre at your leisure...