How to increase the availability of web apps and websites?

Your question is a little vague.

Same as glial. Perhaps adding support for Google Gears or Mozilla Weave once it’s finished?

Multiple redundant servers, preferably with geographic separation between them and round-robin DNS or load balancers. More information than that requires a more specific question than that.

For a comprehensive resource on high availability (HA) web apps and servers, check out
High Scalability

They list the server architectures for Twitter, Google, Amazon, and other websites.

Good Luck!
R

There are so many ways to make server architectures more robust that it is (as others have already said) really hard to answer. The simplest answer is to go with a good co-location company, like iNetU. If you don’t have the people in your organisation who can answer this question already, you may be better off contracting with someone like them, someone who is expert in HA networks, servers, RAID configurations, redundancy, etc., etc.

You might want to take a look at Amazon Web Services

They have storage, database, payment and other products that seem to be very unexpensive and highly reliable.

If you would add more details to your question it would be easier for us to really get an idea of the kind of project you’re referring to, and the requirements it could have.