There's limited (and undocumented) support for boolean queries in Spotlight. After experimenting a little, I found the syntax for OR and NOT.
- lenin|trotsky will find documents mentioning either Lenin or Trotsky
- lenin(-stalin) will find documents mentioning Lenin, but not, thankfully, Stalin
- lenin|trotsky(-stalin) will find documents mentioning either Lenin or Trotsky, but not Stalin
[robg adds: This is the first I've heard of using a minus sign to get a NOT functionality directly in the Spotlight bar. The comments on the linked site (note that it was very slow to load when I tested it) add a bit more detail...]

