Here is some wedding guest etiquette I have picked up from family members, websites, friends, and personal experience as a not-so-good wedding guest.
Definitely
If the invitation says your name “+ guest”, you’re only supposed to bring a date if it is someone you are dating. The bride and groom should have met the date you are bringing or at least know your date’s name. A +1 invitation isn’t an opportunity to bring a random person so they can get drunk with you, even though it’s tempting (and awesome). If you’re bringing a date just to bring a date, you’re just costing the bride and groom money and guaranteeing a guest they don't know appears in the background of all their wedding photos.
Unless you know the bride and groom really well and know something specific they want or are super-crafty and can make something amazing, it is really appreciated when guests buy wedding presents off the registry. Brides and grooms register for items for a reason (probably because according to movies you get a scan gun at a Bed Bath and Beyond and get to go wild and it’s awesome), so help ‘em get the stuff they want and need. Plus there will always be older guests who don't shop from a couple's registry, so if you’re under 60 and can use a computer, you should do so. Even if you find it pretentious that your friend who can’t cook now apparently needs two stainless steel cherry-pitters.
Probably
Most wedding guest etiquette guides say that a guest should not wear white to a wedding. This is probably a good idea so as not to upstage the bride, but a cute white eyelet dress with a cardigan or a man wearing a light seersucker suit could be appropriate at a casual summer wedding.
Depends
A wedding guest etiquette tip I have read recently says to refrain from posting Facebook photos of a wedding until after the bride and groom and their families have posted some and that doing so is like "scooping" the married couple on their big day. Tons of people post facebook photos of a wedding even while the wedding is happening and, honestly, the bride and groom won't get to see their official photos that were taken by a paid photographer until weeks after their wedding, so they may appreciate getting to see a few candid pics their friends took at their wedding in the meantime (also they will be on their honeymoon anyway, so shouldn't get too mad).
Wear
Semi-Formal Afternoon: Dress, Shoes, Clutch, Earrings. Dress code from theknot.com.
Beach Formal: Dress, Sandals, Clutch, Necklace. Dress Code from theknot.com.
Black Tie: Dress, Shoes, Clutch, Scarf . Dress code from theknot.com.
Buy
Gifts Made in West Virginia: 1. Handblown Glass Pitcher and Matching Tumblers by Ron Hinkle 2. Blenko Water Bottle in Cobalt 3. Blenko Tangerine Bowl 4. Large Platter by Anna Brown (you can find all these things and more beautiful pottery by Morgantown potter Anna Brown at WV Market in Morgantown). Check out WV Markets in Morgantown and Snowshoe for more made-in-WV stuff!
Pricier Gifts: 1. Le Creuset Cast Iron Set 2. Pottery Barn Vintage Blacksmith Pot Rack 3. Fiestaware Large Oval Platter (also made in West Virginia) 4. Pottery Barn Hotel Silver-Plated Champagne Bucket (you can make this gift extra-special and get it engraved with the couple's initials)
Budget Gifts: 1. Pottery Barn Caroline Frames (This is a cheaper but thoughtful gift because you don't have to get an expensive frame, but you can put a little extra time into the gift by putting a photo of the couple in the frame. This way they can display it immediately upon opening instead of putting it in a closet somewhere and waiting to get prints of their wedding photos and finding the right size to fit in it.) 2. If you really don't have money to spend on a gift and it's someone you're close to, volunteer to watch their dog while they go on their honeymoon or help assemble dreaded wedding favors.
When in doubt just get drunk and have fun. The bride and groom will probably be too busy worrying about everything to notice a single person. Anyone have a funny wedding etiquette tip they learned the hard way?