I've been using the free Web app Evernote for business purposes for a couple of years. Evernote lets you clip any electronic file (documents, spreadsheets, web pages, photos, etc.), add notes, comments and tags, then keep it on their servers for free, forever.
There is an Evernote app available for just about every smartphone and browser plus client applications for Windows and Mac that extend the functionality and make it easy to type, sort and organize notes into separate notebooks.
I'm now using Evernote to store my kids' medical details: innoculations and vaccines, notes from doctor visits, medication and dosage information, any hospitalizations, surgeries, etc. Once entered, I can pull up this critical information even if we're in a remote city on vacation, or if my daughter is at summer camp and needs to have records sent to a local doctor. [Or while chatting with the school nurse. — Ed.]
There is a paid version of Evernote with some extra features, but so far the free account has been fine for me.
While I think the threat is extremely small, it's wise to consider security and privacy. Evernote has encryption and security protocols in place, but it's still something to keep in mind..
If using a Web app for this purpose makes you uncomfortable, other Parenthackers have suggested record medical details in a spreadsheet and/or storing it on a portable thumb drive which you keep with you. Or, you could keep notes in your phone's notepad (most mobile phones have one, including non-smartphones). And, of course, there's the old pen-and-paper route!
Other ideas? How do you keep your kids' medical details handy?
Related: Keep a list of domestic details on your cell phone or PDA (my favorite: clothing- and shoe sizes)