Currently I have a standard Gmail account. Each site has email forwarders set up and the mail gets forwarded to that Gmail account, and I can reply with different email profiles.
Due to the forwarders not working always reliably, I'm considering using Google Apps for Business.
Is that solution suitable for a few sites across domains working out of on Gmail account?
Would it be compatible with apps that send out emails from my servers (such as forum software and Wordpress)?
My reasonably large company (read 100+) used gmail for it's mail server and it was fantastic. We had several different domains that we had to hook into and you could also hook in your personal email (which I'm not so found of).
As far as I can tell, gmail is compatible with servers that send out emails, like Wordpress, but you should check on that.
As Steve said, there is the free Google Apps version which is good for up to 10 users.
We use it and it works great.
One thing I would be cautious about though is mixing your "corporate site" with your "product site".
E.g. if your product is for instance a big gaming forum that ties into the mail servers, there might be a risk that your forum gets hacked or abused at some point in time and used for spam. This could blacklist your email accounts with Google and other ISPs.
The last thing you want is to have your business email accounts blacklisted.
As such I would keep your corporate email/apps on one domain with their own Google Apps install and your "product" on a separate Google Apps account.
I work in a building with a company that is a Google Apps authorised re-seller and being in the same building as them, they switched over our small company to Google Apps and we haven't had any issues. We use Google Apps for email, documents, time management and pretty much every aspect of project/business management.
Make sure if you do migrate and you're migrating old emails and whatnot that you get someone who's done it before to do it or you might be walking into a sea of troubles if something is messed up.
I'm using the free version Google Apps for email and even it has been enough for my little business. I can specify a custom domain name, and I'm actually using it for two domain names now. I'm also sending SMTP through gmail's servers and all of this has been working great.