You may not need all these items for every tasting, but this is a fairly complete list that should work for most events.
Note cards. Basically, you need to provide a method for guests to take notes. This can be either formal or informal. Formal choices might include the BJCP Checklist score sheet, while an informal solution is a simple one-page rating form listing all the samples along with spaces for notes on appearance, aroma, flavor and overall impression/notes. If you pre-print the forms, you can include reference information (name of beer, brewery, style, price, where purchased, age, etc.).
Pencils/pens. Not everyone will bring writing utensils. Have enough for everyone.
Glassware. Personally, I prefer to use hard plastic tasting cups (5 to 8 oz.) over glassware. Glassware often has an odor from the dishwasher, and can break. Hard plastic cups can crack, but they usually don’t have a plastic odor, they are reusable and they are quite clear. Cleanup is much easier with plastic cups. You can wash and reuse them, or simply recycle them.
Dump buckets. This isn’t wine; you don’t spit when tasting beer. However, not everyone will like every sample, and some might want to control their consumption.
Openers. Most craft beer does not use twist-off caps, so have openers handy. Some beers are corked, so you may wish to have a corkscrew available as well.
Beer (duh). If you forget to bring this, your guests won’t be happy.
Palate cleansers. Provide relatively bland crackers or bread during the tasting of the beer. These aren’t to eat as much as they are to clean the residual flavors from your mouth.
Food. If you aren’t doing a beer-and food-pairing event, you should provide some food to help absorb some of the alcohol and keep people more engaged. Be careful about passing food that will overshadow the beer being served, or save the food for breaks.
Water. Make sure it doesn’t taste of chlorine or other off-flavors. Buy bottled water if your water doesn’t taste good. Use this for both palate cleansing and general hydration while drinking.
Descriptions of beers. Guests will want to know about the beers you are serving. Provide descriptions from the breweries, or from online reviews.
Style guidelines. These might be useful for geekier audiences. BJCP Style Guidelines can help describe the typical characteristics of the beer styles you are tasting.
Table mats. Strongly consider providing a pre-printed form with the names of the beers you will be tasting. At a minimum, provide blank white paper so guests can identify their beers. You will be serving the beers one at a time, but guests will likely want to keep around some of their samples for comparison. Having a place to put them means that the samples won’t get mixed up.
Ample room. Needless to say, you should have room enough for guests to have all their samples on the table at once. If you can arrange the room layout so people can look at each other while they discuss the beers, it will be more of a social format. You can setup the room lecture-style, but this will make it seem more formal and stuffy. You can even do this standing up, provided you have enough side tables where people can put their samples and write their notes.