There are approximately 80 million web searches made every day.
70% of then are made on Google, the biggest search engine of them all.
Around 24% of then are made on MSN, but with a recent re-launch and major marketing spend from its owner (microsoft), this could rise.
Other important Search Engines are
Ask Jeeves - supports real language expressions
Yahoo - originally a directory rather than a search engine, but times change.
Alta Vista - old and reliable - great for MP3 searches.
There are a few more that get used by a significant number of people, but there is no real need to worry about any other than the top 10 or so.
Getting your site submitted to (or registered with) the top ten search engines is important.
However, getting listed does not give you a guarantee that our website will necessarily rank well for any particular search terms.
Danny Sullivan of Search Engine Watch advises "Think of it like a lottery. Search engine submission is akin to you purchasing a lottery ticket. Having a ticket doesn't mean that you will win, but you must have a ticket to have any chance at all."