I got rid of my landline a few years ago, and haven't looked back. One thing I don't miss is the endless calls about "investment opportunities" and solicitations to donate money to various causes. I even received several amusing calls that claimed I had won a prize (I just had to pay a fee to collect it). All of this stopped when I put my landline number on the National Do Not Call Registry. (Note that cell phones are apparently not required to join the registry, though I bet that's just a matter of time.) Here's a brief list of places you can go to stop the madness:
Credit Card Solicitations: OptOutPresceen.com
General Info: read the Federal Trade Commission's Unsolicited Mail, Telemarketing and Email: Where to Go to "Just Say No" (Note: links above are from this page)
If you know of another way to opt out of marketing messages, please leave a tip in the comments and I'll update this post.