Although you're not usually obliged to bring any gifts to the party, it would be a fine gesture to do.  Much depends on the range of the parties and participants: whether it is large or small, for close friends or unfamiliar people, a corporate or a youth party and etc.

You should take all these moments into consideration, while thinking of a party gift to present, or a necessity to give it. Sometimes the hosts of the party tell their guests to bring some stuff. Official large parties accept any party gifts.

Alcohol and food are the most beloved guests at the most parties. Your friends will be always glad to see you in the doorway with a bottle of wine (liqueur, martini, champagne, whisky, etc) and a box of sweets, or even some gorgeous art work of your cooking skills. You may also bring some specific sorts of green tea or exotically flavoured coffee, fruits, cookies, cakes, blue cheese, or anything else of that kind. These probably would be rather not named party gifts, but some kind of your contribution to the party. You may let the mistress know in some unobtrusive manner that she isn't supposed to put all the stuff you brought on the table.

It's a good old tradition in many countries to give flowers to the hostess and drinks to the host. You can replace a bottle of whiskey by Havana cigars, of course if you are quiet sure about the tastes of the host.

If the party isn't official, you can bring some funny stuff like fireworks as a party gift, colourful  masks, costumes, anything that shines in the dark, some tricky things from those tricky stuff shops. Just mind where you go and whom you gonna present party gifts. A good party gift will be a CD with the music according to the occasion and company. Candles and aromatic sticks will do as the small non-obligating souvenirs. If you have any doubts, remember that good wine and pretty flowers will always be apropos.