Sincere generosity is almost always appropriate. The only time when you should think twice about a gift you want to give is when it might create an uncomfortable obligation for the recipient.
In the wedding world, where gifts, gifts, and more gifts are often seen as an entitlement by the Happy Couple and a painful obligation by their guests and friends, it may be more helpful to outline when gifts are not expected. An unexpected gift can, of course, be perfectly appropriate! But both guests and couples should understand the difference between a near-obligation and a delightful surprise.
Engagement gifts are not at all traditional or expected. In the old days, guests were invited to an ordinary party or dinner, and the engagement was announced as a "surprise," so there was no way that guests could come prepared with guests. Close relatives might use the engagement as an occasion to make a special gift, perhaps of jewelry for the bride to wear on the wedding day. But friends and more distant relatives did not give engagement presents.
If you prefer to give a present, or you suspect that you'll be the only person at the party who didn't bring a gift, small tokens are most appropriate from all but the closest relatives. We will hope that the couple's first impulse after getting engaged was not to run out and register for gifts, so there will be no pressure on guests to buy "from the registry."
The Dollar Stretcher provides so many ideas for inexpensive and touching gifts that anyone looking for a delightful "token" gift might as well start with their main tips page. While I'll be the first to admit that saving money is not the only worthy goal in life (or at least, my credit card companies would say so), "frugal" gifts can start you thinking about what kind of present would provide a meaningful memory as well as a nice object.
A shower is a gift-giving party. With a few important exceptions, any guest who accepts the invitation is expected to come armed with a gift. The exceptions are the attendants throwing the party (their costs in hosting the party can count as their gift), parents of the bride and groom (who may already be involved in a great many costs for the wedding), and any guest who attends more than one shower. The rule is one shower gift per guest, so a guest who accepts invitations to two showers would not be expected to bring a gift to the second one.
A shower is not a mandatory part of the wedding festivities, and many brides don't have one. Including registry cards in shower invitations is still generally seen as unacceptable. Guests are not required to buy from the registry. Only guests invited to the wedding are invited to an "official" shower, but no obligation is created if co-workers spontaneously decide to hold a shower at work. Guests who turn down the invitation need not send a present.
The Dollar Stretcher provides a collection of inexpensive, fun, and touching wedding shower ideas.
Only guests who accept the invitation are expected to send or bring a gift. I often hear claims that a Happy Couple invited 300 people, including mere acquaintances and third cousins, as "a gift-grab." If they truly expect 300 gifts, I hope they are ready to feed 300 people, as guests who decline the invitation traditionally do not send a present. (They may, if they like, and relatives often do. But it's not obligatory.)
Technically, even guests who attend are not absolutely required to provide a gift. Most guests do. They have a full year after the wedding to do so, if they wish to. There is also no requirement at all for guests to buy from the registry or to spend a certain minimum amount. Guests should choose gifts that they believe the Happy Couple will enjoy, and that they find comfortably generous. A $10 gift from a perpetually broke pal should be valued just as much as a $500 gift from a wealthy relative.
It is kind to send a card to remember friends' anniversaries, especially if you were a member of their wedding party. Gifts are not customary unless there is a party in honor of a major anniversary: 25th, perhaps 30th, 50th, and 75th.
If your taste runs to giving charming trifles on all sorts of occasions (a level of creativity I always intend to rise to, and never do), you can gain anniversary inspiration from the traditional designations of the anniversaries from the World Almanac. I'd definitely stick to the "traditional" designations, as the "modern" ones suggest "Lifestyles of the Rich and Famous," as interpreted by the Home Shopping Network.