Gmail - After looking at the web-based interface on and off for a while, I decided to take the leap. My primary e-mail address, which uses my own domain (swackhammer.net) automatically forwards all e-mail to my Gmail account. Advantages I love include speed, ability to quickly search all e-mails for what I need, and integration with all my contacts.
Google Voice - I give out one number to everyone, then can customize what phone will ring and when based on who is calling me. Annoying call from recruiter or telemarketer? Just tell Google Voice to send them to voicemail. Or better yet, play a message that indicates your number is no longer in service. :-) And when you do get a voicemail, you can read a transcript of it via SMS or in your e-mail so you don't even have to listen to it. (Although some people's accents make for some very interesting transcripts.)
Google Contacts - Integration with Gmail and Google Voice--all your important contacts in one place, all easily reachable from any web browser.
Google Reader - RSS (Really Simple Syndication) feed-reader allows me to sign up for all the news and blogs I care about and read them at my leisure. I also use the NewsRack app on my iPhone which syncs with Google Reader. Any article I read on my iPhone gets marked as "read" so I won't waste time reading it a second time if I'm using Google Reader in a web browser.
Blogger - I've heard many people say they like WordPress better, but until I need features that WordPress offers, this works great for me.
Best of all, these services are FREE. I know, I know--you may be one of those people that hate Google and don't want them tracking your every move. I'm aware of my online footprint, and as a techie I fully understand that if someone really wants to find out more about me, they will anyway.
How do you use Google? What non-Google services do you love in place of these and why?