To give a bit of background, I am starting off a new festival site so I am particularly interested in Paypal / ticket selling plugins and a mapping plugin for getting people to the festival. The thread is pretty open though, as any good plugin deserves a mention.
2. Headspace 2 - Best SEO plugin ever
3. Gravity Forms - Best form plugin ever. Magical.
4. Q and A - best FAQ plugin ever.
5. Remove Default Canonical Links - Removes canonical link tag which wordpress generates by default.
After installing Gravity forms, you can install Paypal Gravity form add-on. Works like a charm.