ukramedia has put together an excellent compilation of useful scripts and expressions for After Effects.  Two videos include a total of 35 expressions that help automate repetitive actions, speed up the production process, and complete actions that would be difficult and/or time-consuming without using code.

Split in to 2 parts, the expressions included are:

Part 1:

01. 00:14 Value
02. 02:11 Layer Index
03. 04:17 Time
04. 05:14 valueAtTime()
05. 06:59 sourceRectAtTime()
06. 09:28 loopIN() & loopOUT()
07. 11:33 ease() & linear()
08. 13:41 clamp()
09. 15:13 toComp() & toWorld()
10. 17:49 wiggle()
11. 18:22 if/else
12. 19:59 length()
13. 23:11 Math.abs()
14. 24:35 Math.round()
15. 25:22 random()
16. 26:18 Math.min() & Math.max()

Part 2:

01. 03:32 Modulus %
02. 05:33 Active & Enabled
03. 08:10 Parent
04. 10:36 Try/Catch
05. 11:52 hasParent
06. 13:20 Else if()
07. 16:20 Function
08. 19:52 sampleImage(
09. 21:27 Time Conversion
10. 23:27 velocityAtTime() & speedAtTime()
11. 27:43 //Comment
12. 29:19 inPoint & outPoint
13. 32:37 smooth()
14. 34:28 lookAt()
15. 35:37 posterizeTime()
16. 36:22 fromComp() % fromWorld()
17. 39:08 fromCompToSurface()
18. 40:05 key()
19. 44:27 marker.key()