The only thing I know that she'd be really happy to have is something my wife and aren't willing to give quite yet (a grandchild). She won't want flowers. Reads books from the library. Has basically everything she wants.
Massage? Manicure? I'm all out of ideas otherwise.
A donation on her behalf to a cause/charity she really believes in? Or maybe just visit with her and tell her how much you love/appreciate her... Not to be corny, but it's been something like 11 or 12 years since my mother passed, and how I wish I could just do that. My wife's mother passed when my wife was 16, so she's really sensitive to being a mother to our kids and just wants them to love her.
EDIT: I'm not implying you wouldn't do that anyway, but just saying don't get too caught up in the material end of it.
How about something to experience together? Bought a sweet Charter Fishing trip for my Dad and Brothers recently and think it will be something we all get to enjoy together. Mother version could be cool historic tours or...
Those fruit bouquet things, Edible Arrangements is one, have been really big hits with my mom. When she was still working, and I lived on the other side of the country, I'd send one to her office. She loved the fresh fruit and how angry it made the other women in the office because their husbands/kids didn't do it. I loved it because I could spend 25 bucks and have something pretty delivered with like 3 hours notice. They also do have a great price range and are pretty customizable.
Massage or spa day trips work well. I'm actually trying to figure that out this year. Last year she got my daughter, so I don't think my typical fruit bouquet will suffice this year. Maybe a casino trip or a gift certificate for her hair stylist.
Robotic vacuum cleaners are big this Mothers' Day.
write down how you feel about her and everything she's done for you. it'll go further than anything you could buy.
I would gift my mom a spa tour or a massage, which every woman loves to have. While reading this I suddenly got remembered a surprised mothers day I celebrated at my daughter's school. The day was full of surprises, you can also check this out on the school site.
Buy your mom a canoe!
Hockey sweater - wait, that's my mom.
Wine? Vodka? Twelve-year-old single malt? Depends on her preferences and how much of a PITA you were growing up. 
Generally, I just ask my mom if there's anything that she wants and won't get for herself.
If that fails, then I go to places that she likes and try to bring back something that I think that she'll enjoy.
I don't know much about women, but I've learned that a gift that you put a ton of thought and effort into (even if you miss the mark and fail) goes way farther than tossing an Amazon gift card at them. 
Appleseed wrote: Hockey sweater - wait, that's my mom.
She spent the better half of her adult years driving my brothers and I to various ice rinks. A hockey fan she is, but she doesn't want a sweater 
I volunteer every year at a benefit for homeless mothers. Catered lunch, gift bag, presentations, crafts with their kids. But me and about 2-4 other photographers set up a backdrop and lights, and we take a photo of them, and Best Buy donates photo printers and paper so they each get a 5x7 right then in a simple plastic frame.
It's fast pace, and sometimes I'll shoot a mom and kid, sometimes I have 10 people wanting a shot together! But we take it in stride, make them feel special, and it's a huge shot to the feels at the end of the day. My only sadness is if I recognize someone from last year, because that means they haven't gotten out of the system yet.
So, my gift to my mom is doing this event, and she loves it more than any tacky present I could buy her. She's always very proud that I do it. So maybe go the charity route someone mentioned?
I moved out and don't come back often.
Kohl's usually has interesting house wares and knick knacks. Throw a couple together in a gift bag with a card. Taking her out to lunch or dinner usually helps as well.
Weird but I got my mom the genetic testing from ancestry. She has almost no knowledge of her family past her folks who were bastards and so we are going to finally figure out what kind of hinz 57 mutts we are.
Canoe deleted
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