I have a Drobo FS. It’s a great backup solution, but not the most media-streamable NAS out there. I needed something that could stream video, starting out in various Apple-unfriendly formats up to 1080p, from my NAS to my iPhone.
First off, I tried the MediaTomb DLNA media server which installs directly on the Drobo — no attached computer required. Paired with the AirPlayer app as the DLNA player, that system worked well enough for sub-1080p video. 
AirPlayer handled all the various video formats very well (including non-Apple formats like AVI & WMV), but the hardware on the iPhone simply can’t handle downconverting 1080p. Enter transcoding.
Transcoding means your media server converts the resolution/bitrate to something else before firing it out to your playback device. So although the video may start off as a 1080p super-high bitrate clip, your media server downconverts it to 720p or whatever your playback device can deal with. The encoding can change too.
Here’s the current wolfpack of transcoding media streaming iOS apps:
VLC Streamer Free 
Good news: it worked.
Bad news: videos to be transcoded have to be selected individually beforehand on the computer, & they are dumped in your Application Data folder as a million small .ts files.
So, the only way to have a transcoded video library with VLC Streamer was to duplicate my entire video library. Deal killer.
Plex Media Server (yep, PMS)
Good news: The Plex interface looks really slick, both for the PMS web interface & their Plex player iPhone/iPad app ($4.99 but Plex is free otherwise). Plex does a really nice job displaying your media, pulling metadata & matching up cover art from various providers. Also Roku has a free Plex channel app, so you can stream video over WiFi to your TV. Neat!
Bad news: Plex doesn’t let you browse a nested folder structure. They used to, but not anymore. Instead they rely completely on metadata. Or you can restructure your folder structure to fit their models. Their “By Folder” view lumps everything together in one gigantifuck mess. My video library doesn’t have metadata & restructuring is not an option. ARRRHHG! So frustrating, so stupid. Pages & pages & pages & pages of Plex users agree.
Air Video
The good news: everything. So many good things, where to begin. First off, there’s no indexing. Install the Air Video media server & app ($2.99), pick a source folder or two, & you’re done. Thumbnails & transcoding are done on the fly & with no temporary storage on the server … unless you want that (nicely done!). You can stream media within your network or outside too, with or without server password protection. You can download converted clips to your iPhone/iPad for later playback, straight from the app.
Bad news: I honestly don’t have much to say. I wish there were permission levels for shares. Nothing else. Air Video wins.
Air PlayIt / Air PlayIt HD
Good news: Very similar to Air Video, & free! Has three preset permission options (admin, kids, public) that can be set on each share — this alone is brilliant. PMS/DLNA developers should take note. Why permission levels aren’t a standard feature on all streaming engines is a sad, thoughtless mystery. The wait once you click play on a video to transcode is nice & short. Air PlayIt also has the on-demand offline conversion option.
Bad news: Took forever to show the folders on the first view through the app – it waits to do indexing until the folder view is first requested, & then indexes EVERYTHING, while you wait… and wait… and … wait ….. it gets a little better on subsequent views, but all the waiting to just navigate folders is painful. I do have a lot of folders nested so maybe it’s a less common use case, but Air PlayIt server seems to be scanning the entire structure rather than just the section of the tree I’m viewing. Something’s very inefficient there.
Other than that, very good app overall, & can’t really complain that much for the price.
StreamToMe
At $2.99, StreamToMe is another very good option. I’m going to let TUAW.com take this one — if I wrote anything, I’d just be paraphrasing their StreamToMe versus Air Video review.
ZumoCast
ZumoCast was bought by Motorola & isn’t available for Apple devices anymore.
Any Others?
Any streaming apps I missed? Let me know in the comments.