"A good documentation repository that just works." It's one of those evergreen needs that never seem to have a great solution; at least I still haven't found anything 'perfect'.
I tried Google docs, and it did not work for me. Too slow to use, poor hyperlinking architecture to reference things in other documents. Google Sites is better, but not exactly good either -- too awkward and inflexible for 'wiki' style use.
Dror mentions Confluence, and it is a good option. It's certainly very capable and can 'scale' with your growth to a near-unlimited size. But Atlassians products just all have not-so-intuitive user interfaces (to me), and require a good deal of customization to really perform at their best.
Fogbugz ' wiki is rumored to get a overhaul in the next version of Fogbugz. Then it might be worth checking out; the current version isn't too great IMHO. The nice thing about Fogbugz is that it has a suite of version control, bugtracking, code review, and wiki all fitted together.
Google Docs is useless as an internal knowledge base. Cumbersome to use. Poor linking between documents. Heavy weight. Versioning is not straightforward. Search is probably decent but we never used it enough to know.
We use Trac's wiki (we're a software company), but I'd consider MediaWiki. These both do all of the things well that are mentioned above. Search is good enough for our needs. The only problem with these kind of solutions is that you have to self-host. (Unless there's a MediaWiki hosting provider, but I don't know of any.)
If the choice is between Google docs and Zoho, I would recommend Google docs because you can build a rough site around your docs using Google sites. This way, you can have a basic "browsing" experience for when new hires or team members join, but the search functionality of Googles will still be available to you.
I've used Google docs, but not as a replacement for a wiki. It doesn't provide the structured nature that wikis do.
For a startup, you can go for the $10 confluence solution, which lets you use it for up to 10 people. I've used Confluence, and it's a good product, and even when you go beyond 10 people the prices are reasonable. The only down side is that you need to host it yourself.