Companies contact me to put links on my sites - is it a scam

I'd approach something like this very logically, with loads of common sense.

First, I'd start by looking at the sites they are asking about. What do you have going on with those sites? Lots of content? Good traffic? Just a placeholder page. Nothing? Basically, think about what value are they trying to get out of it? That'll give you a hint whether it sounds too good to be true.

Second, I'd see if I could find anything about the company at all, or any of their related names.

What exactly do they want to put on your site?

They could be completely legit, simply looking for advertising opportunities.

They could be trying to set up some kind of back-linking network, or some kind of advertising network.

You could consider offering a trial run on a single domain without revealing the rest of your domains until you are satisfied with what they are doing.

Consider the worst that could happen, where you could be vulnerable. How would you get paid? Make it some way that they can't get access to your critical information. In some situations you may even be able to find an escrow service to go as a go-between, if the transactions end up large enough.

Keep in mind, that unless you do something to hand over the keys to your store (metaphorically speaking), you really have the control here. You are the one with the website, and can control their access, and they are the ones who need to pay.

And finally, if anything smells fishy, bail! (There will always be other opportunities...)

The first thing I do is search for their email address via google within quotes. Sometimes you can find others who were in a similar situation and you can then find out if its a known scam or not. I received emails in the past asking to post links on my sites, I looked up their email address and saw a lot of other people received the same request from that same email and it was considered spam/scam.

I have people contacting me sometimes to advertise. They sometimes ask if I have any other sites they can advertise on too. I usually just say I don't have any others. Asking for a list of your other domains is a bit much though. It's none of their business really.

It depends, but yes there are lot of companies that approach bloggers for a link, review. banners etc. Ive sold both links and banner spots to companies directly. It works.

What looks suspicious to me is they are asking for a list of your domains. Dont they know what sites they want to advertise on? Why else would they contact you?