Just about any sizable web-available service generally needs to run background jobs. This almost always includes some interaction with inbound or outbound mail services which tend to be slow (even with my man Exim on the case). Kinda curious what solutions HN readers prefer for handling this. E.g. a lot of Rubyists use BackgrounDRb, but I prefer my little Looper.rb.
cron. Should exist on every UNIX system, the most common syntax is standardized (you can even call it a DSL if you want), there are multiple implementations, it supports complex command lines, its jobs can be written in any language, it is well documented, and its limitations and security are well understood.