HELP! What kind of dog is this?

There is definately Jack Russell Terrier in there, can tell by the markings and I'm thinking maybe some fox terrier also.
What a cutie pie[]

Do Jacks come in brindle? I didn't know that....cool.

My JRT mix has the exact same spots in the exact same places. The colouring looks really kool on the white coat.

It looks like a Jack Russel or Rat Terrier to me.  Maybe there's Italian Greyhound in there too?  It's just a guess trying to explain the long legs, but I'm probably wrong.  He's got pretty long legs for a Jack, though it's possible.  I've seen brindle in quite a few rescue Jack Russels and I don't think there's any restictions on color as long as they are more than 50% white.  He's a cute little thing, I'd love to see more pictures! 

Brindle in Jacks and Parsons is an automatic disqualification.  There is also no DQ regarding percentage of white.  However, ones that have quite a bit of color are often put behind ones with more white.  The ideal markings are said to be restricted to the head and tail region.  However, many with more color have done well in the show ring.  There is probably a bit of JR there, but I see chi or toy fox terrier.  The bones are too fine for a JR.  In your other post, the first picture, I really don't see much JR in it.  Very cute dog though!

I'd say chihuahua, just not a very typey one. Chihs *do* come in brindle and it's not even terribly uncommon. 

Original =JRTCA Breed Standard for the Jack Russel Terrier
White should predominate (i.e., must be more than 51% white) with tan, black, or brown markings. Brindle markings are unacceptable.

 
I was half right I suppose.  I'd never heard that brindle was a DQ, I guess I just didn't look that far into the standard.  This standard is for the working Jack Russel, I don't think the Parson Russel standard has any mention of the amount of white.  Most of the dogs coming through rescue are quite a ways away from standard anyway, so although it is important to be familiar with the standard, we can't necessarily assume that if the dog is out of standard it is not of the same breed.  Very good points though, Colleen, I'm learning a lot about a breed that I've had quite a bit of contact with, but had never really done much research on. 

Parson's are actually the original "Jack Russell."  Jack Russell really refers to a huge range of terriers that all look different.  Just google Jack Russell and see the different things.  They have now come up with a standard for the "Russell Terrier" which is the longer bodied type.  But, they changed the name to Parson to keep the breed true to its original standards.  The mention of white for Parsons is that they should be predominantly white.  I didn't mean to say that since it didn't meet the standard it wasn't a Jack, but no matter how far from the standard a dog is, there are certain things that will be there.  Such as more bone. 
A note on that site...not very reliable in my opinion. 
"Jack Russell Terriers are a type, or strain, of working terrier; they are not pure bred in the sense that they have a broad genetic make-up, a broad standard, and do not breed true to type."
That is a quote from their "breed history" page.  The real reason there is such a huge variety is that when they were being developed, people would cross random dogs, not keeping true to the breed.  That is how the brindle markings got introduced, by breeding them with bully breeds. 
The thing to remember is that Parson's are the original breed type.  Their conformation is for fox hunting.  They are built for speed and agilty.  The short legged generic Jacks you see are not.  They would not stand much of a chance in a fox hunt compared to the original Parson.  If you want to do some more research, I suggest visiting [linkhttp://www.prtaa.org]www.prtaa.org[/link].  They have a ton of great information.

Here's some more pics of her!

 

 

I think she looks VERY Toy Fox Terrier...here's a picture I just Googled...
 

 

I see either Toy Fox or Rat Terrier.  Both of them have much lighter bone than Jacks.  I would be a bit more likely to say a Rat Terrier mix as this is a very common mix.  How much does she weigh?  Toy Fox Terriers are very tiny.  Rat Terriers look very similar, but are bigger.  Here is a rat terrier.