For instance, Apple (AAPL, Fortune 500) said iCloud would let users store and access documents, photos and media online for free — exactly like Dropbox. Apple’s new iMessenger app will allow users to send group texts over data networks — like GroupMe. And iOS 5 will allow Web surfers to sync bookmarks across their devices through a new feature called “Read Later” — just like Instapaper.

Of course, Apple is far from the first company to incorporate into its own product services that users previously relied on third party developers for.

Last week, Twitter announced it would launch its own photo sharing service, essentially eliminating the need for popular service TwitPic. And after Microsoft (MSFT, Fortune 500) released its own free antivirus software, it become the antivirus market share leader in North America, according to a study released by software management company OPSWAT.

Still, some developers are not pleased.

Camera+: “VOLUME BUTTON TO SNAP A PIC… WHAT AN AWESOME IDEA, APPLE!” tweeted Tap Tap Tap, the maker of the iOS application Camera+, which was told by Apple that it couldn’t use the volume button to let users take photos. Apple unveiled that very feature Monday.

Tap Tap Tap then conceded: “Eh, at least we’re not @marcoarment right now,” referring to Marco Arment, the creator of Instapaper.

Instapaper: In a blog post, Instapaper’s Arment acknowledged that those who use Apple’s new Read Later feature are “prime potential Instapaper customers.” But he described himself as “tentatively optimistic,” saying that the user base of Apple’s new feature may try to upgrade to Instapaper, which he claims offers enhanced features.

“Reading List is either going to have no noticeable effect on Instapaper, or it will improve sales dramatically,” Arment concluded.

Sparrow: Hoa Dinh Viet, the founder of Mac e-mail client Sparrow, took a decidedly more sarcastic tone. During Jobs’ keynote, he tweeted “BREAKING: Apple annonce iAngryAppDev for iOS,” one of a series of cranky tweets about the similarity of many new features to existing applications.

When CNNMoney asked whether Apple’s new Mac mail client, which incorporates many features currently available in Sparrow, would impact the company’s business, Dinh Viet responded with another more subtle jab at Apple: “Will still be able to connect to new iCloud formerly known as MobileMe formerly known as dot Mac mail accounts.”

MobileMe — a previous attempt at linking Apple devices to Web-based services — was not one of Apple’s finest moments, a fact Steve Jobs acknowledged himself in Monday’s keynote address.

Dropbox: Some companies, like Dropbox, are fielding the Apple onslaught by touting their ability to be used across multiple devices and platforms — not just Apple’s.

“There is some overlap, but the two services aren’t really comparable; iCloud is primarily aimed at making iOS devices & iTunes more convenient,” said Drew Houston, CEO and co-founder of Dropbox, in an e-mailed statement. “There’s a big world beyond Apple, and our users love that Dropbox works just as well with your Android phone or PC as with your iPad or iPhone.”

Readability: Like Dropbox, online reading application Readability says that while Apple is focused on itself, Readability has much broader ambitions.

“Apple is laser-focused on delivering a great experience around its hardware and software,” said Rich Ziade, founding partner of Readability, in a blog post. “Our motivation is entirely different: allow our users to read anywhere, on any device, while creating a sustainable ecosystem for writers and publishers.”

GroupMe: Others claimed to be tickled by Apple’s apparent nod to their success.

“We’re honored that everyone keeps thinking about GroupMe in regards to the iMessenger announcement,” Jared Hecht, GroupMe’s co-founder, said in an e-mailed statement. “Fact of the matter is, we’ve always strayed away from being ‘just another messenger.’ We believe the way to do that is by truly being platform agnostic.”