Today, a number of online services are trying to solve the problem of note taking on the fly. They give people using a computer or mobile device a central place to save notes, links and photos. This week, Google joined the already crowded field by introducing Keep, a service that is clearly inspired by its rivals. Google's effort is inevitably a serious one, if only because it has such big name behind it and access to hundreds of millions of existing users.

Given the number of note taking services available, figuring out which one is best can be difficult. They all have strengths and weaknesses based on their features, design and compatibility. The differences between services are in the details.

Can you use your voice to create notes instead of having to go to the trouble of typing them? Is an app available on Android devices? How much data can you upload per month without having to pay? In the end, a note taking service that works for one person may not for another. Here's recap of five note-taking services and what they offer.

Evernote

With 50 million users, Evernote is the pack's leader. The service, available on iOS and Android, has most everything users need like the ability to organize information into individual notebooks and share with others. To avoid having to type notes, Android users can use an automated transcription service that turns speech into text -- at least in theory. However, there is no built-in equivalent for iOS users. Evernote's free service limits uploads to 60 MB per month. A premium version with up to 1 GB of uploads per month costs $45 annually. Earlier this month, Evernote suffered a black eye when it revealed that hackers had infiltrated its computer system. As a result, it had to reset all user passwords.

Simplenote

A close rival of Evernote, Simplenote offers many of the same kinds of features, but limited to iOS devices and the Web. Unlike the others, Simplenote's free service comes with ads. A premium version without ads and extra features like synching with Dropbox, the online storage service, is $20 annually. In a sign of how hot the note-taking field is, Automattic, the company behind WordPress blogging software, acquired Simplenote's parent earlier this year.

Notability

Notability is known as a palette for annotating notes and files stored on the service. Users can highlight text, write using their finger on documents including PDFs and draw pictures. Annotated files can be saved and shared. Notability is available only as an iPad app, which costs $2. Because of its limited compatibility, the service is not for everyone. Schools and people in creative professions may find it highly useful (and fun).

Catch

Catch covers most of the bases necessary in note taking service. Text, audio and photo notes can be organized into folders and tagged so that they can be easily found. Users can also create checklists, share information privately and collaborate, with each contribution listed so you know who added what. Catch's free service, which limits uploads to 70 MB per month, is available on Android and iOS devices along with the Web. Premium versions with bigger uploads and extra sharing -- aimed largely at business customers - cost $45 and $145 annually.

Keep

Basic is the best word to describe Google's foray into note taking, which is part of its Drive online storage service. Keep has a simple design and offers voice to text transcription and sharing. But compared with some rivals, it lacks some functions like the ability to categorize notes and set an alarm for reminders. It is also only available on Android 4.0 devices or higher and through the Web. Google has a reputation for introducing skeletal products and then improving them over time. But more recently, it has also developed a reputation for killing products that don't work out, which doesn't exactly build confidence among users who have been burned. In fact, this is Google's second try at note taking after killing a previous effort, Notebook, last year.