Xara is a vector based 2D drawing package made by Xara Group Ltd. Although not designed for making web graphics in the first place, to me it's an excellent web graphics design tool. Because it's vector based you can scale your drawings without quality loss. Another great feature is the use of anti-aliasing, not limited to fonts. This prevents jagged edges.
Note: if you consider buying Xara Xtreme Pro I strongly suggest to read Xara Xtreme Pro is severely crippled first.
Xara is well known for its transparency features. Many nice effects can be accomplished with it.
Although Xara is a vector drawing program, it can handle bitmaps as well. It even has basic image processing, and a trace tool. The latter makes it possible to turn a bitmap into a vector drawing. Export to various vector and bitmap formats is supported.
I have written several small Xara tutorials, the so called Xarials. I hope to add some more in the near future. Contact me if you are interested in updates.