Hi Leon,
If you shot in RAW, Adobe's ACR (Adobe Camera RAW) which comes with Photoshop has a leveling tool(Edit: Called Straightening Tool, 7th icon from the left at top of screen, keyboard shortcut: A), Lightroom might have it I think(Being almost identical in features to ACR)
*Edit: Ok, have found it in Lightroom if you are using that: Got to Tools --> Crop (keyboard shortcut: R) and you can either grab a corner of the image and drag it to change the angle, or use the angle slider in the tool window. The grid overlayed on the image will help. Not as nice as ACR's 'Straighten tool' with which you simply click on one side of the horizon and drag a line to the other side... very easy.
Edit again(!) I have just discovered that the straighten tool does indeed exist in Lightroom(I'm new to Lightroom), click the little Spirit Level icon in the Crop tool window. I didn't realise that was clickable, thought it was just a little graphic to show what the slider does!
http://www.nathangriffin.com/technic...lightroom.html
If however it is a JPG image, you can use Photoshop's Ruler tool to level:
http://www.thephotoargus.com/tutoria...op-ruler-tool/
There is also the Trimming/Angle Adjustment tool in Canon's Digital Photo Professional, but I find it fiddly to use. Not a huge fan of Canon's software.