It's always handy to keep a few go-to gifts for those last minute needs.  Having a standard present can also help when you get gift brain block.

As host/hostess gifts, something consumable is usually good.  Depending on the person, you could bring a bottle of wine, some fancy coffee, a small basket of yummies (check out the bins at World Market, I'm just sayin'), some lovely candles.  Of course, if you craft or cook, you can't beat handmade candles or soaps or preserves.

Weddings are always a challenge, especially since people are marrying later and often have fully equipped households.  Glassware (either regular glasses or barware) is handy just because things break so easily.  Picture frames are quite handy as well.  On the more unique side, consider experiences.  You can make it a habit to find a museum in the place where the couple lives and buy a membership.  You can consider theater or symphony tickets as well.  (Memberships are great gifts for familes as well.  Plus, you can often download and print out a certificate at the last minute.)

For babies, I like to crochet a blanket or cross stitch a bib or a receiving blanket. When I have no craft projects in the queue, I make blankets and bibs so I have some in stock.  If you aren't crafty, onesies, T-shirts, and crib sheets are always useful.  Having extras means less laundry!

What other events do you need to stock presents for?