I came across Advanced Diary, and I was extremely surprised at just how much it was exactly what I was looking for. Here's some of the key features that I really like, in terms of a diary.
"Wordpad"-like: I was expecting something like notepad: write down text, but not much formatting. I was wrong. Advanced Diary has it's own little "Wordpad" built in, filled with formatting like font type, font size, font color, bold, italics, underline, highlighting, alignment, "find in text", tables, hyperlinks, bullets, numberings, spellcheck, and more *GASP FOR AIR*. That's alot of features for just a diary! Correct me if I'm wrong, but that's actually more than Wordpad! It might not be as much as Microsoft Word, but it definitely has enough for a good diary.
Password protected: This is a must, in my opinion. Journals are often very personal, and you don't want people snooping around, so having a password is a necessity for a good diary program. But then, it's also an optional feature, for maybe if you are just journaling about something non-private, like your favorite sports team.
Calendar: It has a wonderful little calendar in month form, which you can browse through. What's even extra cool is that the day is bold if it has an entry, meaning you can tell at a glance what days have entries and what days don't.
"Find": This actually surprised me about how good it was. Say you remember journaling about a certain word, like "freeware", but couldn't remember what month, much less what day, you had mentioned it in. Advanced Diary has a wonderful search that will pull up every entry that has the search terms in it.
Link to other entries: This is really kewl. Say you refer to something you said last week, and you want to provide an easy way to find where you said it in your Advanced Diary journal. You can actually create a hotlink, just like to a URL, except it will take you to that Advanced Diary entry. (Keep in mind, Advanced Diary is not tabbed, so you can't multitask, but it's still a useful tool.)
Autosave: If you've ever used Google's Blogger service, you know how useful this is. You'll be typing along, your browser/internet/computer will crash, but you don't worry about typing that long post, because Blogger auto-saves the draft as you go along. Advanced Diary has the same functionality, only better. You can actually specify how often you want it to auto-save, meaning it can save every 1 minute, assuring you that no entry will ever be lost.
Images: Maybe you want to illustrate your journal with doodles or pictures. Advanced Diary can do that too. One of the nice things about it is that (I believe) it actually copies the picture data into the Advanced Diary file, not just the link to the image file. That means that if you delete the image on your hard drive, it won't dissapear out of your diary, like in some other word processors *coughOpenOfficecough*. On the downside, there aren't as many formatting features for images as there are in processors like MS Word or OpenOffice. You can't choose the wrap (how the text moves around the image) or really any other formatting, but you can adjust the alignment (just like with text) and resize.
Thesaurus: This is kind of a bonus, but you can select a word and use the built in thesaurus, which you can then use to replace the word with a synonym.
Multiple Diaries: This is an interesting feature. Suppose you want to journal, but want to keep it "separated", like maybe journal about school in one place, and then about your latest gaming venture in another, and so on and so forth. Advanced Diary actually lets you create multiple diary's in hierarchical form. For example, your diary list might look something like below:
Overall, it's a fascinating concept. It's really a good way to further organize your entries.
Print, Backup/Restore, Import/Export: Need I explain? I shall. You can print the current entry, all entries in the current diary, or all entries from a specified date range. You can backup for safety, and then restore if something goes awry. You can import (which is actually really picky and depends on what the files are named), and you can export in RTF, TXT, or HTML (images will actually be exported along with the HTML file!). Overall, it's a great way to get entries in and out of Advanced Diary.
It really is a wonderful utility. Some might say "It's just a glorified, organized notepad, with password protection!" And I would respond ".....so?" What more could you ask of in a Diary?
There is one catch though. Unfortunately, the makers of Advanced Diary decided that it's so good, it should be worth $30. However, the last free version available (version 1.3) is available through the ever-amazing and wonderful A+ Freeware, and can be downloaded from thus. This entire blog post was about the free v1.3, so don't think you are being cheated of any features.