like zaiah said, there is 1 validation error, but that you can sort out in no time at all ... the error basically states that there is no "target='_blank'" in XHTML 1.0 strict, so either switch to XHTML 1.0 Transitional or find a workaround for that.
The design is clean, but I must say that it looks a bit 90's imo ...
One thing that bothers me a bit is that on the "home" page, the left hand side menu is longer than the content. it's not a big thing but it is a bit disturbing.
Also on the rollovers inside the left hand side menu the color and background-color are too close to each other, which makes it impossible to read the link that I am hovering on.
maybe it's just me, but on the "home" page there is that "random image", but on wider screens it really gets a bit too random just floating there all the way to the right. Maybe make so that the page is always a certain width and in the center of the browser window?
Also then, maybe make the link of the page that a user is on unclickable, for instance if I am on the "animals" page, make the "animals" link unclickable and in another colour (or rollover state), so that a user know which page he/she is on, and doesn't redirect to that page again while he/she is on it ...
last thing, and this is about saving a little bit of bandwidth, if you remove the spaces and line-breaks from the code you'll save a few kbs, now let's say for this example that you save 5kbs on the "home" page, and let's say that you get 200 visits a day to your site. so that makes a total of 1000kb a day saved in bandwidth, which may not seem like a lot, but then you can use the bandwidth available to you more efficiently.
Let's leave all our *plum* where it is and go live in the jungle ...