1. Daring Fireball ($2,750 / rss text ad) – John Gruber over at Daring Fireball has created a cult following with over 150,000 subscribers. These people are mac and iphone owners who have a high standard and will look upon your application with high standards. If you manage to impress John and his fans, you could potential influence very influential people.
2. Fusion Ads($950 / month) – Fusion ads is a growing network that relies upon exposing readers of a handful of popular industry blogs one ad per page. You’re ad will see a lot of viewers, but the topics of all the blogs vary and you may wander outside the realm of your target audience.
3. The Deck ($7400 / month) – The deck operates almost exactly like Fusion Ads, except on a grander scale. Your ads will be seen on very popular blogs like A List Apart (need I say more.) The price tag is a lot heftier too.  You might want to check out the special iPhone deal which I believe would get you views in only iPhone applications like Twitterific.
4. 148Apps- This site is specifically about iPhone app reviews, so your audience is not only catered to iPhone users,  but the audience will be people interested in possibly purchasing an iPhone application.
5. Google Adwords – I’d like to omit this option from the list, just because everyone knows about it. Nevertheless, advertising images can get you ads seen on sites like Macworld.com, Macrumors.com, Lifehack.org and Cnet.com.
6. MacThemes ($150 / months) – You’ll want to advertise strictly on the forum because the blog has come to a stop. The forum has momentum still and advertising will expose you to a younger audience that are strictly iPhone fans.

I’ve given a good list to look at when advertising, but let me break down why advertising can be effective. Say you spend $20 per day to get 10 downloads per day of your application (these are just dummy numbers, don’t read into them). At this point you’re losing over $10 from this activity, but if 10 downloads are enough to get you on the top 100 downloaded in a app store category then you’ll increase your visibility on the charts. If landing on the charts can generate 15-20 sales then you’ll actually see a profit because of the indirect causes of advertising. If you think about, the app store is a fixed fight when you look at the charts. All application don’t receive the same promotion, so you could rank better if you spent more marketing dollars on an application.